The proliferation of long term evolution (LTE) bands has engendered a need for raising the number of single-band filter chips to about 30 in high-end phones. However, as the number of filters in the radio frequency (RF) module increases, the module footprint area also increases. Further, shrinking the size of the filter die is limited as the silicon (Si) area is fixed by impedance matching or, rather, the reduction in size is driven by packaging.
In addition, Si-made MEMS oscillators show decent performance, but they only operate at frequencies below 1 gigahertz (GHz). Aluminum nitride (AlN) resonator oscillators are suitable for high frequencies, but they suffer from thermal drifting that is too large for timing applications and too small for thermal sensing applications. Thermometric beat frequency sensors using a conventional MEMS might solve some problems, but 1) they have an uncertain thermal gradient between the resonators, which decreases sensitivity and accuracy, 2) they have an increased footprint, 3) they have process variations larger than the target accuracy (i.e. variations in resonator sizes), 4) the MEMS use overtone modes which have uneven energy and low electromechanical efficiency, and 5) they have restricted applications and accuracy due to frequency separation between the modes.
A need therefore exists for methodology enabling manufacture a dual-mode MEMS resonator and a dual band acoustic filter suitable for all LTE bands, having a reduced footprint area, and resulting devices. A need further exists for an MEMS oscillator which operates at high frequencies with thermal drifting small enough for timing applications yet large enough for thermal sensing applications, having a reduced footprint area, and resulting devices.